This invention relates to electrical wiring for a motor vehicle and, more particularly, to electrical wiring for a motor vehicle having a primary system which includes a voltage-controlled generator, a primary energy accumulator as well as at least one primary energy consuming device, and having a paralleled secondary system which includes a secondary energy accumulator as well as at least one secondary energy consuming device.
Vehicle electrical wiring of this type is known from German Patent document DE 38 41 769 C1. In the case of this known vehicle wiring, the primary energy accumulator, specifically the starter battery, and the secondary energy accumulator, specifically the wiring battery, are connected with one another as soon as the generator becomes operative. When the generator current supply is low, this has the disadvantage that the wiring battery is charged by charge compensation at the expense of the starter battery when the charge condition of the wiring battery is poorer than that of the starter battery.
In the case of the known wiring, a switch is also mounted between the wiring battery and the consuming devices of the wiring, i.e., secondary energy consuming devices, and the remaining portion of the wiring. When the generator stops, this switch will open up as soon as the starter battery voltage falls under a permissible limit value. This measure protects the starter battery, i.e., the primary energy accumulator, only when the generator is stopped with respect to a complete discharge and, therefore, permits a partial discharge of the starter battery. At the start of the drive, the load to which the starter battery is subjected will increase when a charge compensation of the two batteries takes place at the expense of the starter battery. In an extreme case, this may lead to a complete discharge or destruction of the primary energy accumulator, specifically of the starter battery.
There is therefore needed an electrical wiring network for a vehicle which always ensures an optimal charge condition of the primary energy accumulator.
According to the present invention, this need is met in that the secondary system includes a control unit including a controllable resistor which is connected to the generator. The control unit monitors the charge condition of the secondary energy accumulator, and, as a function of the actual charge condition of the secondary energy accumulator, controls the controllable resistor and connects and disconnects the secondary energy consuming device or devices. This is done in such a manner that, on the one hand, during the charging of the secondary energy accumulator, the primary system is not overloaded and, on the other hand, during the discharge of the secondary energy system, the charge value does not fall below a predetermined residual charge value.
Since the primary system containing the generator, as a function of the electric load of the wiring, that is, practically, of the primary energy consuming devices and the operating condition of the internal-combustion engine driving the generator, cannot provide sufficient electric energy at all times, the wiring according to the present invention offers the possibility of controlling, to a certain extent, the charging current to the secondary energy accumulator and thus the loading of the primary system. In addition, through the use of the controllable resistor which influences the charging current to the secondary energy accumulator, the primary system is damped, whereby the harmonic characteristic of the wiring is reduced. However, it is a prerequisite in this respect that the secondary energy accumulator can take up a sufficient amount of current; that is, the secondary energy accumulator is not already completely charged.
According to a preferred embodiment of the present invention, the primary energy accumulator is arranged closer to the generator relative to the secondary energy accumulator. This has the result that the electric specific resistance between the generator and the primary energy accumulator is lower than the electric specific resistance between the generator and the secondary energy accumulator. Thus, during the charging of the two energy accumulators, the voltage drop on the feed line to the primary energy accumulator is lower than the voltage drop on the feed line to the secondary energy accumulator. This has the result that the charging of the primary energy accumulator always has priority and, therefore, always has an optimal charge condition.
If, for example, the starter of the motor vehicle internal-combustion engine is one of the primary energy consuming devices of the wiring according to the invention, then by always having an optimal charge condition of the primary energy accumulator, maximum startability, and hence the use of the motor vehicle, will always be ensured.
In an advantageous further embodiment of the invention, the current charge condition of the secondary energy accumulator is determined starting from a predetermined or predeterminable initial charge condition by the addition of the charge quantity supplied during the charging operation as well as by the subtraction of the charge quantity removed during the discharging operation. The efficiency of the secondary energy accumulator is also taken into account.
In this case, the charge quantity fed to the secondary energy accumulator is determined by the time integration of the charge current.
Analogously, the charge quantity removed from the secondary energy accumulator is determined by the time integration of the discharge voltage relative to the load resistor or to the sum of the load resistors.
When the wiring becomes operative, the electric charge contained in the secondary energy accumulator, that is, the charge condition of the secondary energy accumulator, is fixed at an initial value. This initial value can most easily be determined by approximation from the off-load voltage and the temperature of the secondary energy accumulator. Advantageously, this takes place in that, during the starting operation of the motor vehicle during which the secondary energy accumulator is preferably connected neither with its load nor with the primary system, the voltage of the secondary energy accumulator as well as its temperature are measured. During the operation of the wiring according to the invention, the charge condition of the secondary energy accumulator is then corrected according to the fed and removed charge quantity, while, in the case of the removed charge quantity, the efficiency of the secondary energy accumulator must be taken into account by means of a factor k.
According to another advantageous further embodiment of the present invention, the charge current is limited by the controllable resistor of the control unit of the secondary system to a maximum permissible current intensity value. As a result, excessive loading of the primary system is avoided.
According to a further embodiment of the wiring according to the invention, the steepness of the rise and drop of the charge current of the secondary energy accumulator is limited to a maximum permissible value by the controllable resistor of the control unit. As a result, interfering electromagnetic radiations are reduced to a minimum.
The controllable resistor of the control unit which determines the charge current of the secondary energy accumulator is advantageously formed by several field effect transistors which are either connected behind one another or in parallel.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.